The invention relates to AM stereo radio broadcast receivers and is particularly directed to a receiver using the Magnavox system recently announced as one of the systems of choice by the Federal Communications Commission. In this system, the conventional amplitude modulated (AM) radio channel carries the L+R stereo signal so that a conventional monaural radio receives a compatible signal. The L-R stereo signal is transmitted as a phase modulation (PM) of the carrier. A subaudible pilot tone also phase modulates the carrier and its phase modulation is substantially greater than that of the L-R component. Since the conventional radio will not respond to the PM, it will not be affected, thereby. However, if a limiter and detector are added to a conventional radio, the AM will be ignored in the added circuit and the PM can be recovered. Therefore, the L-R information and pilot signal can be separately recovered. It is then only necessary to matrix the two channels to recover the stereo signals for reproduction in a stereo audio system. While the circuits disclosed herein are intended for use with the proposed Magnavox system, it is to be understood that the functions performed can be used with other proposed AM stereo systems.
A copending patent application Ser. No. 197,294 filed Oct. 15, 1980, by Don R. Sauer, is titled AM STEREO PHASE MODULATION DECODER and is assigned to the assignee of the present invention. A PM decoder is disclosed for use with the Magnavox system receiver.
A copending patent application Ser. No. 187,006 filed Sept. 15, 1980, by Don R. Sauer, is titled FULL WAVE AMPLITUDE MODULATION DETECTOR CIRCUIT and is assigned to the assignee of the present invention. It discloses a detector circuit primarily intended for use in the AM portion of AM stereo receivers and has applications to the present invention as will be described hereinafter.
A copending patent application Ser. No. 200,636 filed Oct. 27, 1980, by Don R. Sauer, is titled AM STEREO RECEIVER SEPARATION CONTROL and is assigned to the assignee of the present invention. Means are disclosed for causing the L-R signal in an AM stereo receiver to track the amplitude of the L+R signal. When the two are matrixed together good L and R separation is obtained even for variable level signals. Also means for electronic blending of L and R is disclosed.
The above three patent applications are incorporated herein by reference.